1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus and a method of driving the same. More particularly, the present invention relates to a solid-state image pickup apparatus of the type sweeping out unnecessary charges existing on the vertical transfer paths before reading out signal charges from photosensitive cells and transferring them to vertical transfer paths.
2. Description of the Background Art
A digital still camera and a video camera belong to a family of image pickup systems using solid-state image sensing devices. Today, in the imaging art, the charge transfer type of solid-state image sensing devices typified by a CCD (Charge Coupled Device) image sensor are predominant over the other types of image sensing devices. Smear and blooming are undesirable phenomena particular to a charge transfer type of solid-state image sensing device. Smear refers to whitish stripe-like noise appearing in a reproduced picture over the entire field in the vertical direction when the luminance of part of a subject picked up is predominantly higher. Blooming refers to an occurrence that when the quantity of light incident to a photosensitive cell or pixel is excessively greater, that cell generates an excessive signal charge which in turn leaks into surrounding photosensitive cells, rendering portions around the cell white.
Other noise components particular to semiconductor devices associated with imaging include dark signal or dark current components, i.e., charges stored little by little with the elapse of time even when light is intercepted, e.g., when the shutter of a camera is closed. The smear components, blooming components and dark signals appear in a picture as fixed pattern noise, and are factors which will degrade image quality.
In light of the above, it is a common practice with a digital still camera of the type controlling an exposure time with a mechanical shutter to execute the following procedure in order to sweep up dark signal components generated on vertical transfer paths and smear and blooming components leaking from photosensitive cells. After the mechanical shutter has been closed, the vertical transfer paths and a horizontal transfer path connected thereto are driven at a rate higher than a usual transfer rate for thereby sweeping up charges from the vertical transfer paths. This processing will be referred to as sweep transfer hereinafter. Subsequently, signal charges are read out from the cells.
To give priority to resolution in a still picture mode to other features, a digital still camera uses either one of a solid-state image sensing device performing progressive scanning and a solid-state image sensing device performing frame scanning. The progressive scanning refers to a procedure that reads out signal charges from the whole pixels in a picture at the same time and transfers them pixel by pixel. The frame scanning refers to a procedure that reads out signal charges alternately field by field from odd-numbered lines and even-numbered lines.
When use is made of the image sensing device performing the frame scanning, two fields of signals are necessary for producing a single picture, i.e., a single frame. Therefore, when a digital still camera produces a single picture by a single exposure, its mechanical shutter must be closed after exposure in order to intercept light for thereby preventing the first and second fields from being varied. In this case, unnecessary charges are swept out after the mechanical shutter has been closed.
The image sensing device performing the progressive scanning allows a single picture to be produced by a single exposure without resorting to a mechanical shutter. However, sweep transfer is essential even with this type of image sensing device because dark signal components are generated in the vertical transfer paths and because smear components and blooming components leak from the photosensitive cells.
It has been customary to assign a preselected transfer frequency and a preselected number of transfer stages to sweep transfer, and therefore to fix a sweep transfer period necessary for dissipating unnecessary charges. The transfer frequency and the number of transfer stages, i.e., the sweep transfer period has heretofore been determined such that sweeping is repeated, e.g., three consecutive times for sweeping out the expected maximum amount of unnecessary charges. Why sweeping is repeated a plurality of times will be described hereinafter.
Assume that an excessive quantity of light is incident to the image sensing device, causing the smear components and blooming components to increase. Then, such an amount of unnecessary charges to be swept out exceeds the transfer capacity of the vertical transfer paths and cannot be swept out by a single sweeping step. As a result, part of the unnecessary charges is left on the vertical transfer paths after the sweeping step.
More specifically, the operator of a digital still camera may shoot a desired scene while watching a picture displayed on an LCD (Liquid Crystal Device) monitor screen, i.e., without viewing the scene through an optical finder. An excessive quantity of light may be incident to the image sensing device when the operator shoots a scene including, e.g., the sun in the daytime while viewing the LCD monitor screen. Consequently, vertical stripes ascribable to the defective sweeping appear in the resulting picture on the monitor screen because the amount of unnecessary charges to be swept out exceeds the transfer capacity of the vertical transfer paths.
The three times of sweeping, for example, have customarily been effected in consideration of the defective sweeping stated above. This, however, sometimes makes the sweep transfer time excessively long in comparison to the amount of unnecessary charges actually stored on the vertical transfer paths, thereby extending the overall image-pickup time for a single picture. Furthermore, the sweep transfer is effected at a high speed and therefore consumes substantial power per unit period of time. In this sense, too, the excessively long sweep transfer period is not desirable.